Propene recovery

ABSTRACT

Process to separate propene from gaseous fluid catalytic cracking products by performing the following steps: a) separating a feed mixture comprising the gaseous products, propene and other saturated and unsaturated hydrocarbons obtained in a fluid catalytic cracking process into a hydrocarbon-rich liquid fraction and a hydrogen containing gaseous fraction, b) separating the hydrogen containing gaseous fraction into a hydrogen-rich gaseous fraction and a hydrocarbon-rich gaseous fraction by means of a membrane separation, c) supplying the hydrocarbon-rich gaseous fraction obtained in step (b) to an absorber section and obtaining in said absorber section a lower boiling fraction rich in gaseous products having a boiling point of ethane or below and supplying the hydrocarbon-rich liquid fraction obtained in step (a) to a stripper section and obtaining in said stripper section a higher boiling fraction comprising propene and hydrocarbons having a boiling point higher than ethane.

The invention is directed to a process to separate propene from theproduct stream obtained in a fluidized catalytic cracking process.

In a fluid catalytic cracking (FCC) process a mixture of hydrocarbons isprepared by means of catalytic cracking of a petroleum distillate orresidue fraction. The hydrocarbon reactor effluent is separated in aseparation section into gasoline, light and heavy cycle oil and gaseousproducts, for example methane, LPG, propene and butene. In aconventional separation section the reactor effluent is first separatedin a so-called main fractionator. The top product obtained in the mainfractionator will comprise next to the so-called permanent gases likehydrogen, methane and nitrogen a certain amount of ethene, ethane,propene, propane and other saturated and unsaturated hydrocarboncompounds having a boiling point of below 220° C. The valuablehydrocarbon compounds boiling in the gasoline range are recovered fromthis top product in a so-called unsaturated gas plant as described inU.S. Pat. No. 4,605,493.

U.S. Pat. No. 4,605,493 describes a process in which the top product ofthe main fractionator is first compressed in one or more stages to ahigher pressure level. This section is also referred to as therecontacting section. After the recontacting section the gaseouscompounds having a boiling point of ethane and below are separated fromthe hydrocarbon products having a boiling point of at least propene andabove by means of distillate separation step. This distillate separationstep comprises in that the compressed top product is separated in agaseous fraction and a liquid fraction by means of a flash operation.The gaseous traction is sent to an absorber section and the liquidproduct is sent to a stripping section. In the combined absorber andstripping section propene, propane and higher boiling hydrocarboncompounds are separated from the gaseous compounds including ethane andethene and lower boiling compounds. In the embodiment illustrated inU.S. Pat. No. 4,605,493 the absorber and stripping sections arerepresented by two separate vessels. Embodiments in which both sectionsare present in one column are also known from the prior art.

Propene has become an important by-product of a FCC unit operation. Theimportance of this by-product is for example illustrated by the factthat dedicated ZSM-5 containing catalyst additives are used to enhancethe propene yield in a FCC unit operation. A problem often associatedwith the increase in propene yield above the design value of an existingplant is that the above described rectifying absorber becomes abottleneck. This bottleneck may for example result in that the part ofthe extra propene prepared in the FCC reactor will not be separated fromthe gaseous products in the rectifying absorber. Recovery of propenefrom the gaseous product downstream of the rectifying absorber iseconomically less attractive.

The object of the present invention is to provide a process in whichpropene can be separated from the gaseous FCC products with a higherefficiency than is possible with prior art processes.

This object is achieved by the following process: Process to separatepropene from gaseous fluid catalytic cracking products by performing thefollowing steps:

a) separating d feed mixture comprising the gaseous products, propeneand other saturated and unsaturated hydrocarbons obtained in a fluidcatalytic cracking process into a hydrocarbon-rich liquid fraction and ahydrogen containing gaseous fraction,

b) separating the hydrogen containing gaseous fraction into ahydrogen-rich gaseous fraction and a hydrocarbon-rich gaseous fractionby means of a membrane separation,

c) supplying the hydrocarbon-rich gaseous fraction obtained in step (b)to an absorber section and obtaining in said absorber section a lowerboiling fraction rich in gaseous products having a boiling point ofethane or below and supplying the hydro-carbon-rich liquid fractionobtained in step (a) to a stripper section and obtaining in saidstripper section a higher boiling fraction comprising propene andhydrocarbons having a boiling point higher than ethane.

Applicants have found that by separating of part of the hydrogen presentin the feed to the rectifying absorber in step (c) that the efficiencyof the propene recovery is improved. This improvement enables one, forexample, to either make use of smaller distillate units for new FCCunits or to de-bottleneck existing FCC units enabling a higher propeneproduction. An additional advantage is that hydrogen is obtained havinga reasonable good quality in a relatively simple manner when compared tomethods which can recover hydrogen from gaseous fractions at a pointmore down stream of the rectifying absorber. A further advantage is thatin step (b) part of the sour gasses like H₂S and HCN are removed withthe hydrogen, thereby reducing the amount of corrosive compounds in step(c).

The feed mixture of step (a) is a mixture obtained in a FCC mainfractionator comprising gaseous products and saturated and unsaturatedhydrocarbons ranging from methane to hydrocarbons having an atmosphericbolting point of about 253° C. and preferably to about 220° C. Apartfrom hydrogen the gaseous FCC products comprise various components likeNH₃, H₂S, CO, CO₂, and H₂O. The feed mixture has a pressure typicallybetween 11 and 25 bars. The content of hydrogen in the hydrogencontaining gaseous fraction will suitably be 3 vol. % or higher. In atypical FCC process the hydrogen content in the hydrogen containinggaseous fraction will be between 5-20 vol. %.

The separation in a liquid and gaseous fraction in step (a) can beachieved by a conventional flash operation, for example in a knock outvessel. It has been found advantageous to reduce the contaminant level(especially NH₃ and H₂S) of the hydrogen containing gaseous fractionsent to step (b). This leads to a more hydrogen-rich gaseous fraction instep (b). Removal of sour gasses can be achieved by conventionalmethods. For example by contacting the feed prior to step (a) with waterand separating in step (a) the feed mixture into a sour water fractionand the above mentioned hydrocarbon-rich liquid fraction and hydrogencontaining gaseous fraction. Such a separation may be suitably performedin a three phase separation vessel.

Step (b) can be performed by making use of conventional membraneseparation means, which are known to be selective of separating hydrogenfrom small hydrocarbons. Selective separation occurs when a pressuregradient across the membrane is applied. Preferably a hydrogenseparation selectivity greater than 20, more preferably greater than 50,is required, wherein the selectivity is defined as the permeabilityratio of hydrogen over methane. Permeability is defined as the number ofmoles of a compound which permeates a membrane per square meters per dayper bar of pressure difference.

It is also advantageous for the propene recovery in step (c) whenmembranes are used through which methane and ethane will permeatesignificantly faster than the heavier hydrocarbons (C₃+). Preferably themembrane has a methane separation selectivity of greater than 5, whereinthe selectivity is defined as the permeability ratio of methane overpropane. Suitable membranes should further have a sufficient permeationrate for the hydrogen and should have a sufficient life time. Preferredmembranes further show a good resistance to liquid hydrocarbons. Themembranes can be made from either inorganic or organic material.Examples of inorganic materials are ceramic, carbon and molecular sievematerials. An example of a ceramic membrane is described in U.S. Pat.No. 5,827,569. Organic membrane materials are preferably of a polymermaterial, for example polyaramid, polyetherimide and polyimid. Examplesof commercial membrane systems which can be used in the processaccording to the invention are Medal of L'Air Liquide, Prism alpha ofAir Products, Polysep of UOP and Membrane Systems (e.g. module B-H) ofUbe.

The membrane is suitably in the form of a hollow fibre placed in amembrane unit in a conventional manner known to one skilled in the art.In such a membrane unit a bundle of hollow membrane fibres are placed ina vessel in such a manner that hydrogen present in the feed to the unitcan pass the membrane fibre from the shell side to the inside of thefibre resulting in a second gaseous fraction rich in hydrogen and agaseous fraction enriched in hydrocarbons. The vessel has outletconduits and spaces to collect the hydrogen rich gaseous fractioncollected in the fibres and inlet means at the shell side for thegaseous fraction and outlet means for the hydrocarbon rich fraction. Ina preferred embodiment a number of such vessels are arranged in seriesin order to achieve the desired separation and to avoid the use of largevessels.

The temperature in step (b) is preferably at least 20° C. higher thanthe dew point of the hydrogen containing gaseous mixture send to step(b) when membrane materials are used which are sensitive to liquidhydrocarbons. Additional means for heating this gaseous mixture shouldthen be provided to heat the hydrogen containing mixture prior to step(b). Preferably the temperature in step (b) is between 50-100° C. andmore preferably between 70-90° C.

The hydrogen containing gaseous fraction in step (b) suitably will havea pressure greater than 11 bar, preferably greater than 15 bar whichenables an efficient separation in step (b). The pressure ratio of thepressure of the hydrogen containing gaseous mixture send to step (b) andthe hydrogen-rich gaseous mixture obtained in step (b) is suitablygreater than 2 and preferably greater than 5. Although the separationrate is negatively influenced when a low pressure ratio is used, it mayin some cases be advantageous when the resulting higher pressurehydrogen-rich gaseous fraction is further purified. When thehydrogen-rich fraction is used as fuel higher pressure ratios mayadvantageously be applied. Preferably more than 50% of the hydrogenpresent in the hydrocarbon feed mixture is separated in step (b).

In step (c) the hydrocarbon-rich gaseous fraction obtained in step (b)is supplied to an absorber section. This absorber section may be asingle column or a combination of more columns which comprise at leastmeans to condense the gaseous top product, means to recycle thecondensed top product to the absorber section and means to discharge ahigher boiling liquid fraction to the stripper section. This liquidfraction may be advantageously send to step (a) in order to separate anygaseous compounds in this fraction before sending it to the strippersection. This latter embodiment is illustrated in U.S. Pat. No.4,605,493.

The absorber section may further be suitably provided with means tosupply a liquid hydrocarbon mixture, which mixture is poor in at leastpropene, to the top or discharge end of the absorber section. Thishydrocarbon mixture, also referred to as lean oil, serves to absorb intothe liquid phase as much propene and other valuable higher boilinghydrocarbons in the absorber section before being discharged to thestripper section. Examples of suitable sources of lean oil are thehigher boiling fraction obtained in a debutanizer or the condensedfraction directly obtained from the top product of the main fractionatorof a fluidized catalytic cracking process. In the condenser the lowerboiling gaseous fraction rich in gaseous products having a boiling pointof ethane or below is obtained.

In step (c) the hydrocarbon-rich liquid fraction obtained in step (a) issupplied to a stripper section. The stripper section is provided withreboiler means to evaporate any lower boiling compounds resulting in agaseous fraction, means to discharge the higher boiling fractioncomprising propene and hydrocarbons having a boiling point higher thanethane and means to discharge the gaseous fraction to the absorbersection. This gaseous fraction may be send to step (a) before beingsupplied to the absorber section as illustrated in U.S. Pat. No.4,605,493. However preferably the gaseous fraction obtained in thestripping section is send directly to the absorber section in order toachieve that the hydrogen concentration in the hydrogen containinggaseous fraction obtained in step (d) is as high as possible. A higherhydrogen concentration is favourable for the efficiency of the membraneseparation in step (b).

The stripper section may be d single column or a combination of morecolumns. An example of an embodiment of step (c) is described in theafore mentioned U.S. Pat. No. 4,605,493. In a preferred embodimentabsorber section and the stripping section are combined in onedistillation column, optionally provided with one or more additionalside-coolers and reboilers. Such a combined column is referred to as aso-called rectifying absorber.

It has been found that the propene recovery is even further improvedwhen the hydrocarbon-rich liquid fraction obtained in step (a) is fed toa position in the rectifying absorber column above the feed inlet of thehydrocarbon rich gaseous fraction obtained in step (b). Preferablybetween 2-6 practical trays are present between these two inlets.

The operation conditions in the rectifying absorber may be thoseconventionally applied. The pressure at the top may typically rangebetween 10 and 25 bars and the bottom temperature between 110 and 140°C.

The higher boiling fraction comprising propene and hydrocarbons having aboiling point higher than ethene obtained in step (c) can be furtherprocessed in a conventional manner in which propene is recovered bydistillation from the other hydrocarbon products.

The invention shall be illustrated making use of FIG. 1A and FIG. 1.FIG. 1A represents a rectifying absorber column according to the stateof the art. Via stream (1) a feed mixture comprising gaseous products,propene and other saturated and unsaturated hydrocarbons is supplied toa knock out vessel (2) resulting in a hydrocarbon-rich liquid fractionwhich is discharged via stream (4) to the rectifying absorber column (5)and a hydrogen containing gaseous traction which is discharged viastream (3) to the rectifying absorber column (5). The rectifyingabsorber column (5) is equipped with a gas outlet (8) for the gaseoustop fraction, a condenser (9) and a condenser collection vessel (10) inwhich the gaseous components are separated via (11) from the condensedliquid fraction which liquid fraction is recycled via (12) to the top ofthe column. Via stream (13) lean oil is mixed with the gaseous topproduct up-stream of condenser (9). Most of the propene present in thegaseous fraction present in stream (8) will be absorbed by the lean oiland returned to the rectifying absorber via (12). The rectifyingabsorber is further equipped with a reboiler (7). Via stream (6) theliquid bottom fraction enriched in propene is discharged to downstreamseparation units.

FIG. 1 represents a process according to the invention. The meaning ofthe reference signs is the same as in FIG. 1A. In addition to theprocess represented in FIG. 1A a membrane separation unit (14) is shownin which a hydrogen-rich gaseous fraction is obtained and discharged via(16) and a hydrocarbon-rich gaseous fraction is obtained which issupplied to the rectifying absorber column (5) via (15).

The invention is especially directed to a method for retrofitting anexisting separation unit which is part of the down stream separationmeans of a fluid catalytic cracking unit, and wherein in the separationunit the gaseous compounds having a boiling point of ethane and beloware separated from the hydrocarbon products having a boiling point of atleast propene. The existing separation unit, which has also beendescribed above, comprises an absorber and stripping sections andseparation flash means in which the hydrocarbon feed is first separatedin a liquid and gaseous fraction. Preferably the absorber and strippingsections are combined in one rectifying absorber column. Theretrofitting comprising adding means to remove hydrogen, preferably bymeans of membrane separation, from the gaseous fraction obtained in theflash separator. Preferably use is made of existing feed inlets in therespective absorber and stripping sections for the gaseous tractionobtained in the membrane unit and the liquid fraction obtained in theflash separator.

The invention is also directed to the use of a membrane separator toremove hydrogen from a feed of a distillate separation unit which isused to separate gaseous compounds having a boiling point of ethane andbelow from hydrocarbon products having a boiling point of at leastpropene.

The invention will be illustrated by the following non-limiting exampleswhich are calculations using a mathematical model describing the knockout vessel and the rectifying absorber. A conventional value for themembrane separation efficiency is used.

EXAMPLE 1

To a knock out vessel a typical FCC compressed top product of the mainfractionator is sent having a pressure of 17.1 bar. Hydrogen wasseparated from the gaseous mixture as obtained in the knock out vesselin a membrane separation unit resulting in a gaseous mixture rich inhydrocarbons. The hydrogen rich gaseous fraction obtained has a pressureof 2 bars. The hydrocarbon-rich mixture is supplied to a typicalrectifying absorber at the same feed inlet location as the feed inletlocation of the liquid fraction obtained in the knock out vessel. Thepropene recovery is 95.1% calculated on the feed mixture. See also Table1.

EXAMPLE 2

Example 1 as repeated except that the feed inlet of the gaseous mixturerich in hydrocarbons is 4 practical trays below the feed inlet of theliquid fraction obtained in the knock out vessel. The feed inletposition of the liquid fraction is the same as used in Example 1. Thepropene recovery is 96.1% calculated on the feed mixture. See also Table1.

EXAMPLE 3

Example 1 is repeated except that the membrane area is half the areaused in Example 1. The propene recovery is 93.3% calculated on the feedmixture. See also Table 1.

EXAMPLE 4

Example 1 is repeated except that the membrane area is 50% larger thanthe area used in Example 1. The propene recovery is 95.8% calculated onthe feed mixture. See also Table 1.

COMPARATIVE EXPERIMENT A

Example 1 is repeated except that the mixture having the composition (I)is supplied directly to the rectifying absorber without making use theknock out vessel and the membrane unit. The location of the feed inletis the same as in Example 1. The propene recovery is 89.2% calculated onthe feed mixture.

TABLE 1 Example → 1 2 3 4 membrane area (M²) 6000 6000 3000 9000Pressure of stream (11) (bar) 16.3 16.3 16.3 16.3 Fraction of H₂ removed(% on 76 76 56 84 feed) propene recovery (% on feed) 95.1 96.1 92.3 95.8capacity increase compared to base   6%  11%   5%   9% case: Comparativeexperiment A

What is claimed is:
 1. Process to separate propene from gaseous fluidcatalytic cracking products by performing the following steps: a)separating a feed mixture comprising the gaseous products, propene andother saturated and unsaturated hydrocarbons ranging from methane tohydrocarbons having a boiling point of 250° C. as obtained in a fluidcatalytic cracking process into a hydrocarbon-rich liquid fraction and ahydrogen containing gaseous fraction, b) separating, at a temperaturebetween 50 and 100° C., the hydrogen containing gaseous fraction into ahydrogen-rich gaseous fraction and a hydrocarbon-rich gaseous fractionby membrane separation means defined by having a methane separationselectivity and a hydrogen separation selectivity, c) supplying thehydrocarbon-rich gaseous fraction obtained in step (b) to an absorbersection, wherein to the top or discharge end of the absorber section aliquid hydrocarbon mixture is supplied to, which hydrocarbon mixture ispoor in propene, and obtaining in said absorber section a lower boilinggaseous fraction rich in gaseous products having a boiling point ofethane or below, and d) supplying the hydrocarbon-rich liquid fractionobtained in step (a) to a stripper section and obtaining in saidstripper section a gaseous fraction and a higher boiling fractioncomprising propene and hydrocarbons having a boiling point higher thanethane.
 2. The process of claim 1, wherein the gaseous fraction obtainedin the stripping section is supplied directly to the absorber section.3. The process of claim 1, wherein the higher boiling fraction issupplied to step (a).
 4. The process of claim 1, wherein the strippingsection and the absorber are combined in one distillation column.
 5. Theprocess of claim 4, wherein the hydrocarbon rich liquid fractionobtained in step (a) is fed to a position in the distillation columnabove the feed inlet of the hydrocarbon rich gaseous fraction obtainedin step (b).
 6. The process of claim 1, wherein the hydrogen separationselectivity of the membrane separation in step (b) is greater than 20,wherein the hydrogen separation selectivity is defined as thepermeability ratio of hydrogen over methane.
 7. The process of claim 1,wherein the methane separation selectivity of the separation in step (b)is greater than 5, wherein the methane separation selectivity is definedas the permeability ratio of methane over propane.